The present invention relates to a brake system for a motor vehicle which has a brake pressure-generating means, with or without power-assistance, with a non-pressurized storage reservoir, the brake pressure-generating means being connected to a human-machine interface, which by way of a brake fluid serves for generating hydraulic brake pressure for brake devices, preferably friction brakes, the brake system providing at least a first hydraulic unit for connecting the brake pressure-generating means to the brake devices of at least one axle and for transmitting the hydraulic brake pressure through the brake fluid, the first hydraulic unit being electro-hydraulically controllable and capable of operation independently of the brake pressure generated by the brake pressure-generating means, and having a pump with a high-pressure side and a suction side.
The brake system specified above is often used for motor vehicles which besides an internal combustion engine also have an electric motor for drive purposes (hybrid drive). The electric motor here can also be used as generator for obtaining electrical energy (recuperation), a braking force generated by the generator being applied during the recuperation to at least one axle of the motor vehicle, preferably the rear axle.
The purpose of such regenerative brake systems in motor vehicles is to store at least a proportion of the energy applied in braking in a battery and to reuse this for driving the vehicle. This serves to reduce the overall energy consumption of the motor vehicle, to increase the efficiency and thereby to make operation more economic.
WO 2004/101308 A1, which is incorporated by reference, discloses a regenerative brake system for motor vehicles, in which the electrically regenerative brake (generator brake) in the ranges to which it is suited is used in parallel with the hydraulic friction brakes. In such working operation of the electrically regenerative brake, however, the pressure consumption balance of the brake fluid of the friction brakes is often upset because, when account is taken of the deceleration achieved, the brake pressure of the friction brakes applied by the brake pressure-generating means is reduced to a lesser extent than in a purely conventional braking with friction brakes, since the electrically regenerative brake does not reduce the brake fluid pressure or in operation does not take up any volume of brake fluid. For this reason, in the known system for braking with an electrically regenerative brake, brake fluid is diverted into a pressure accumulator, which is preferably embodied as a low-pressure accumulator, in order thereby to simulate an additional braking of the friction brake. In this way the pressure of the brake fluid or the pressure volume is reduced, which in turn has an effect on the brake pressure-generating means (brake pedal). A comfortable braking sensation is thereby achieved.
DE 10 2007 036 859 A1, which is incorporated by reference, discloses a brake system in which an isolating valve, which is of a type that remains open in the absence of a current, is arranged between the tandem master cylinder and the inlet valve of the friction brakes on the axle with recuperative braking. This isolating valve on the one hand cuts off the hydraulic connection to the tandem master cylinder and on the other, in the currentless emergency state, allows a direct hydraulic-mechanical action on the associated friction brakes. The signal for closure of the isolating valve is generated by the detection of driver braking. Isolating the brake pressure-generating means and the friction brake and/or hydraulic unit makes it possible to control the brake device of the corresponding axle on by-wire principles (brake-by-wire, electromechanical control). A pump, which supplies the necessary system pressure, is provided in the by-wire brake circuit.
A direct hydraulic action on both axles of the vehicle is generally afforded in the event of a failure of the electronic control of the hydraulic brake circuit, so that a maximum utilizable deceleration is achieved on road surfaces with a low coefficient of friction, since all wheels are capable of transmitting braking force up to the limit of grip.
In order to isolate the hydraulic unit from the brake pressure-generating device, the isolating valve of the known brake system is energized and is thereby closed from the time braking is detected until the driver ceases to brake. Between the individual braking operations the valve ensures volume and pressure equalization via the tandem master cylinder reservoir.
In order to achieve a dynamic pressure build-up during the by-wire control, the known system moreover comprises a converter, which is arranged in parallel with the pump and has two chambers, that is to say an inlet-side control chamber and an outlet-side supply chamber, which are separated from one another by a moveable, spring-biased piston. The converter supplies the pump on the suction side with an inlet pressure, which ensures a good supply of brake fluid at the required pressure, even at low temperatures, and regulates this. The hydraulic volume and transmission ratio of the converter is here selected so that there is sufficient priming pressure and intake volume available for the pump.
The known system with such a converter has the disadvantage, however, that it is costly to manufacture. Moreover, in terms of the response time it does not yet satisfy the stipulated requirements, since the priming occurs only with a delay. The known system furthermore has the disadvantage that the priming causes a loss of delivery capacity, since the volume delivered into the volume accumulator for the priming cannot be used for building up brake pressure.